This invention relates to spread-spectrum communications, and more particularly to code-division-multiple-access (CDMA) cellular, packet-switched communication systems. The inventive concepts involve optimization of packet data communications using a hybrid DSMA-CR/CDMA multiple access method with a second level collision resolution.
Recent developments in wireless communications technologies have allowed expansion of service offerings from the original voice telephone service model to include a number of services supporting packet data communications. As customers become increasingly familiar with data services offered through landline networks, they are increasingly demanding comparable data communications in the wireless domain, for example to maintain service while mobile subscribers roam freely or to provide remote service in locations where wireless loops are preferable to landline subscriber loops. A number of technologies support packet data communications in the wireless domain.
For example, a random access channel (RACH) provides packet uplink transport from a mobile station (MS) to a base station (BS), with a random slotted-ALOHA type access procedure. A common-packet channel (CPCH) provides a similar uplink transport for transmitting variable size packets from a mobile station (MS) to a base station (BS). The RACH and CPCH channels do not need direct resource allocation. The channel resource-allocation of these channels is contention based. The mobile station transmits an access preamble corresponding to a channel that the mobile station desires to use. The base station responds with a matching preamble that signals successful access to the selected channel resource.
The CPCH also utilizes a first level collision detection phase, to allocate the channel to a mobile station that successfully avoids collision. If two or more mobile stations are still attempting access to the same channel at the time of the collision detection phase, the base station will respond with at most one matching collision detection preamble, effectively allocating the channel to the one mobile station. In some cases, the base station will not be able to resolve the collision detection and will not send back any collision detection preamble. A mobile station that fails to receive its matching collision detection preamble from the base station aborts its access attempt.
A Digital Sense Multiple Access (DSMA) implementation of a common-packet channel (CPCH) provides additional mechanisms, such as the status broadcast and the piggybacking to the basic CPCH. In a Digital Sense Multiple Access (DSMA) system, whenever a base station detects the presence of a subscriber unit transmission on the reverse channel it asserts a periodically occurring flag, called a xe2x80x9cbusy/idlexe2x80x9d flag, on the associated forward channel. This flag is asserted logically true whenever the channel is busy. Any subscriber unit that already is transmitting when the busy/idle flag is set true may continue to transmit. However, all other subscriber units desiring access to a channel must wait. Essentially, each of the mobile stations will interpret a busy state as an instruction to xe2x80x9cbackoffxe2x80x9d and delay its next access attempt or wait until the busy/idle flag is reset or cleared indicating that at least one channel is idle and available. Since the two-way propagation delay is much less than the minimum packet length, the DSMA type protocols perform much better than traditional slotted ALOHA type protocols. The physical layer and the underlying spread spectrum system allow the quick detection of a collision. This approach only allows the base station to generally throttle back the traffic flow.
These wireless packet data communications do not adequately address collision resolution, instead of detection, to various undetected contending mobile stations. Also, there are gaps in channel utilization due to non-arrival probability. For example, more than one mobile stations may be attempting to access channel 1 at time t while no mobile station is attempting to access another one of the channels on the CDMA system, leaving the other channel vacant. At a later time t+1, there may be no mobile station at all attempting to access the same channel contended for at time t, leaving that first channel vacant.
It has been found that there is roughly a 30% chance that two or more preambles from mobile stations will arrive at the base station in any given 50 ms time-window. There also is a 32% chance that no mobile station requests access to a particular channel during the same 50 ms time-window. These collisions and gaps in channel utilization effectively limit system throughput and block further improvement in system throughput.
This invention introduces a second-level collision resolution method and apparatus, which addresses the above issue, thus improving the attainable throughput gain. The second-level collision resolution method provides an approach to resolve collision and concurrently allow the base station to assign a channel resource out of a predefined group of channel resources to one or more contending mobile stations. The method also allows priority schemes.
Hence a general objective of the invention is to allow mobile stations to request access to a group of packet channel resources. The request selection can be based on availability of the channels within the group of packet channel resources.
Another objective relates to improving data throughput by having the base station assign channel resources from the requested group to various mobile stations, and thus facilitate a more even distribution of usage over available channels.
The present invention provides an improvement to a code-division-multiple-access (CDMA) system employing spread-spectrum modulation. Mobile stations initially seek access to a selected one of a plurality of groups of channels serviced through a base station. A principal feature of the invention relates to a second order collision resolution phase, which also serves to allocate one or more available channels from the group among contending mobile stations.
In a preferred embodiment, the CDMA system has a radio network controller (RNC) and a plurality of base stations, which serve a plurality of mobile or remote stations. Each base station (BS) has a BS-spread-spectrum transmitter and a BS-spread-spectrum receiver. Each mobile station (MS) has an MS-spread-spectrum transmitter and an MS-spread-spectrum receiver. In the preferred embodiment, the RNC monitors traffic demand, based on traffic measurement information of communications through the base stations for the mobile stations. Based on the traffic demand or a projection thereof, the RNC assigns channel resources to the base stations, by re-configuring the channel resources within each cell.
A base station broadcasts, on a periodic or non-periodic basis, availability related status information for two or more groups formed from the channels allocated to the base station. For example, the status information can contain actual availability information, i.e. idle or busy of one or more channels in each group, or available data rate information for each group, or both. At a mobile station, the steps include monitoring the broadcast(s) of the status information. Based on the broadcast status information, the MS selects a group of m channels containing at least one idle channel. The channel group selection by the MS can utilize a dynamic persistence algorithm or any other algorithm commonly known in the art.
Following channel group selection, the MS starts transmission of a series of access preambles (AP). Each AP contains a signature selected from a set of predefined signatures used for communications with the particular base station (BS). The selected AP signature corresponds to a predefined group of m channels. The MS transmits the access preambles at well-selected time intervals and at increasing power levels. The MS stops its transmission of the access preambles when the access preamble has been picked up and detected by the BS, the BS has responded with an acknowledgment AP-AICH, and the MS has also successfully received the AP-AICH. Alternatively, the MS ceases its access preamble transmissions if the MS has transmitted the maximum allowed number of access preambles MAP.
In the inventive system, the AP-AICH corresponds to the preamble used for requesting access to one group of channels. At this point, the base station has indicated that there is at least one channel in the group actually available, but a specific one of the channels has not yet been assigned to or seized by any one mobile station. In some cases, two or more mobile stations are seeking access to channels in the same group. Sometimes there will be enough available channels to service all of the mobile stations requesting access, and sometimes there may still not be enough available channel resources. In accord with the invention, specific channel assignment to one or more individual mobile stations occurs as part of the collision resolution phase, so that this phase performs a second order collision resolution function as well as a resource allocation function.
Upon receiving this AP-AICH signal, each mobile station seeking access to the group of channels randomly selects one collision detection (C/D) signature from a predefined signature set and transmits a CD preamble containing that signature. Since each mobile station randomly selects one of the possible collision detection (CD) signatures, typically, no two stations seeking access to the same group will pick the exact same CD signature.
When the base station receives one or a plurality of CD preambles from a plurality of mobile stations, it identifies all of the different CD preambles from the received signals. The base station then selects only n mobile stations. Here, n is the number of available channels in that channel group. The base station transmits a CD acknowledgement (CD-AICH) signal to each of the n mobile stations. Each CD-AICH corresponds to the CD preamble signature selected from the received signals and refers to one assigned channel from the group of m channels. Assuming successful resolution of any overlaps, and selection of the n CD preambles, the base station has effectively assigned the n available channels to the n mobile stations by sending back the appropriate CD-AICH signals.
Upon receiving a CD acknowledgement CD-AICH, which corresponds to the sent CD signature, the mobile station begins to send its packet data along with any closed-loop power control information. For this transmission, the mobile station uses the channelization codes for the one channel of the group referred to by the particular CD-AICH signal received from the base station. The base station also sends its downlink closed-loop power control information simultaneously. A pre-data power control phase is optional.
If the mobile station detects a loss of the downlink channel, for example, during transmission of the power control preamble or the packet data, the mobile station halts its uplink transmission. Essentially, the mobile station aborts the access attempt and sends a failure message to the MAC layer of the associated data equipment. The base station can utilize this feature, by cutting off the downlink transmission, to instruct mobile stations not to use a channel following an unresolved collision.
During a transmission of data, the mobile station that has successfully obtained access can piggy-back packets, one after another, so long as it has packets ready to send, up to a maximum limit set by the network. Essentially, this allows the mobile station to hold the channel if the MAC equipment supplies further packets to the PHY elements in the midst of an uplink transmission.